diff --git a/README.md b/README.md
index cd82042..8370b0b 100644
--- a/README.md
+++ b/README.md
@@ -9,7 +9,7 @@ and pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_876 TILs and counting..._
+_877 TILs and counting..._
 
 ---
 
@@ -551,6 +551,7 @@ _876 TILs and counting..._
 - [Show Pending Migrations](rails/show-pending-migrations.md)
 - [Show Rails Models With Pry](rails/show-rails-models-with-pry.md)
 - [Show Rails Routes With Pry](rails/show-rails-routes-with-pry.md)
+- [Test If An Instance Variable Assigned](rails/test-if-an-instance-variable-was-assigned.md)
 - [Truncate Almost All Tables](rails/truncate-almost-all-tables.md)
 - [Update Column Versus Update Attribute](rails/update-column-versus-update-attribute.md)
 - [Where Am I In The Partial Iteration?](rails/where-am-i-in-the-partial-iteration.md)
diff --git a/rails/test-if-an-instance-variable-was-assigned.md b/rails/test-if-an-instance-variable-was-assigned.md
new file mode 100644
index 0000000..f68998b
--- /dev/null
+++ b/rails/test-if-an-instance-variable-was-assigned.md
@@ -0,0 +1,24 @@
+# Test If An Instance Variable Was Assigned
+
+When testing Rails controller actions, you'll often be making assertions about
+the response to your test request. You may also want to assert about the
+instance variables being set, as those are headed for your view layer.
+
+For instance, if an instance variable `@metadata` is supposed to be set in a
+`#show` controller action, you can assert that it is with [RSpec's
+`assigns`](https://relishapp.com/rspec/rspec-rails/docs/controller-specs):
+
+```ruby
+describe "when given valid params" do
+  it "sets the metadata" do
+    get :show, params: valid_params
+
+    expect(assigns(:metadata)).to match(
+      identifier: "abc123",
+      session_id: "fe98f08c-bf2f-4749-9f81-071d9cc7720e",
+    )
+  end
+end
+```
+
+[source](https://stackoverflow.com/questions/2051373/how-to-test-instance-variable-was-instantiated-in-controller-with-rspec)